1. Field of the Invention
The present invention relates to woven fabric articles, and more particularly to multi-ply woven fabric acoustical panels of the type which have controlled porosities for admission of sound wave energy into compartments formed within the panel where the energy is dissipated.
2. History of the Prior Art
Three-dimensional, integrally woven fabrics have been found to provide the basic superstructure for acoustical panels of low cost capable of meeting the demands previously satisfied only by very expensive acoustical materials. In the field of aircraft design, for example, three-dimensional woven fabrics when resin impregnated for rigidity have been found to provide relatively inexpensive acoustical panels which can be mounted in critical areas of jet engines and other areas of the aircraft so as to absorb and dissipate substantial amounts of sound. Such acoustical panels typically have a plurality of internal ribs which are integrally woven with opposite faces so as to form compartments therein. At least one of the faces, and in some cases selected ones of the ribs, are provided with a plurality of apertures appropriately dimensioned and configured so as to admit and thereby trap sound waves within the compartments from outside the face.
The apertures within the various plies of the acoustical panel play a key role in its sound deadening capabilities. On the one hand at least one of the faces must be provided with a sufficient number of apertures of sufficient size so as to pass a considerable volume of sound waves to the inside compartments for dissipation. By the same token the faces and the other portions of the acoustical panel must have the necessary strength and rigidity so as to be useful for the particular applications in question.
A common problem in making a panel which is rigid strong and yet adequately apertured arises from the fact that the yarns used to weave the faces and interconnecting ribs of such fabrics undergo considerable flattening during weaving of the fabric and during subsequent resin impregnation thereof. Thus where the various faces and interconnecting ribs are comprised of single piles of the woven fabric, the fabric cannot be woven too loosely if it is to have a reasonable amount of strength, rigidity and structural integrity. By the same token weaving of the fabric in relatively dense fashion so as to provide such suitable properties is often accompanied by considerable difficulty in controlling the porosity of the woven fabric plies. With the fabric densely woven to begin with, the accompanying flattening of the yarns which occurs during weaving and during lay-up and cure often results in most or all of the apertures between adjacent yarns of the fabric being completely covered over with resin.
A number of techniques have been employed to enable the weaving of single ply faces and ribs so that they have both high density and the required amount of porosity. One such technique is disclosed in U.S. Pat. No. 3,481,427, Dobbs et al., issued Dec. 2, 1969. In the Dobbs et al patent at least one of the faces of the acoustical panel is woven of high-twist yarns. The high-twist condition of the yarns increases their density and thereby their resistance to flattening. In an alternative technique described in a copending application Ser. No. 268,475, filed July 3, 1972, Acoustical Panel, Leon Parker, now U.S. Pat. No. 3,756,346 which patent is commonly assigned with the present application, the critical areas of the acoustical panel are comprised of standard low-twist yarns having other low-twist yarns served or braided therein so as to greatly increase their resistance to flattening.
By employing techniques of the type referred to in the Dobbs, et al patent and in U.S. Pat. No. 3,756,346 it is usually possible to provide an acoustical panel which not only has the necessary acoustical properties but which has considerable strength and rigidity. However it may be desirable for certain applications to provide panels of this type having greatly increased strength and rigidity. Moreover for reasons such as cost or manufacturing technique it may be desirable to provide alternative techniques for making such panels. For example it would be advantageous in most instances to be able to greatly decrease the complexity, cost and time required for the weaving operation such as by being able to weave a relatively low density fabric, while at the same time providing the resulting acoustical panel with a selected, highly controlled porosity. It would also be desirable to be able to readily make certain portions of the woven fabric porous and other portions of the fabric non-porous in an effective and efficient manner, without the necessity for resorting to such techniques as variation in density during weaving or in the types of yarns used.